1. Field of the Invention
The present invention relates to an image forming apparatus and a recording material conveying method, which conveys a recording material to a fixing unit that fixes an image held on a recording material, as well as a program for implementing the method and a storage medium storing the program.
2. Description of the Related Art
Conventionally, a fixing unit mounted in an image forming apparatus is comprised mainly of two fixing rollers. One of the two fixing rollers has a heater incorporated therein. This heater comprises a heat roller 901 (see FIG. 40) that generates heat when energized. It should be noted that a cylindrical film may be used in place of the heat roller. The other of the two fixing rollers comprises a pressurizing roller 902 that is pressure-cotacted urging contact with the heat roller 901 so as to form a nip section in the area of contact between the two fixing rollers. FIG. 40 is a diagram showing the axial direction-wise temperature distribution in the nip section of the conventional fixing unit.
A recording material that holds a toner receives heat and pressure when passing through the nip section of the two fixing rollers, and the toner is fixed on the recording material by the heat and pressure. Based on temperature data from a thermistor, the fixing unit controls the fixing temperature so as to ensure a fixing temperature necessary and sufficient for the toner to be properly fixed on the recording material.
The size of the recording sheet passing through the fixing unit varies from a relatively large A3-size to a relatively small postcard size. Thus, depending on the size of a recording material, the recording material is in contact with some areas of the nip section but is not in contact with the other areas of the nip section. In the case where recording materials of any size are caused to pass through the midsection of the fixing rollers in the axial direction, the thermistor for controlling the fixing temperature is usually disposed in the midsection of the fixing rollers in the axial direction. Referring to FIG. 40, a first thermistor 903 is disposed in the midsection of the fixing rollers in the axial direction. Also, a second thermistor 904 is disposed at an end of the fixing rollers in the axial direction.
The reason why the second thermistor 904 is disposed at the end of the fixing rollers is as follows. If no recording material is present in the fixing unit, the axial direction-wise temperature distribution of the nip section is substantially uniform (see FIG. 40). This is because the heater is configured to uniformly generate heat in the axial direction. Conventionally, such a heater is widely used for fixing units so as to reduce costs, enhance ease of control, and realize high durability, etc.
FIG. 41 is a diagram showing the axial direction-wise distribution in the nip section when a recording material passes through the midsection of the fixing rollers. When a recording material 910 passes through the midsection of the fixing rollers, heat is drawn from the midsection by the recording material 901, and therefore the temperature in the midsection decreases. On this occasion, the first thermistor 903 detects the temperature decrease in the midsection, and hence the quantity of electric current passed through the heater is increased to generate much more heat so as to keep a predetermined fixing temperature. On the other hand, no heat is drawn from the ends of the fixing rollers by the recording material 910, and hence the temperature at the ends of the fixing rollers (hereinafter merely referred to as “the end temperature”) does not decrease. That is, the distribution of temperature is such that the temperature is low in the midsection of the rollers and is high at the ends of the fixing rollers.
FIG. 42 is a diagram showing the axial direction-wise temperature distribution in the nip section in the case where recording sheets are continuously passed through the fixing unit. In the case where recording sheets are continuously passed through the fixing unit, no heat is drawn from the midsection of the fixing rollers in the interval between a certain recording material and the next recording material (i.e. sheet-to-sheet interval), and hence the temperature in the midsection does not decrease, and the heater generates only a small amount of heat. Also, the end temperature does not increase due in part to the shift of heat at the ends of the fixing rollers shift to the midsection where the temperature is low. As a result, the temperature distribution becomes nearly flat.
As described above, the end temperature tends to increase in the case where the width of a recording sheet is smaller than the axial width of the fixing rollers. It is known that when the end temperature exceeds a predetermined upper limit, this will cause a failure of the fixing unit. Accordingly, a scheme to prevent temperature increase at the ends of the fixing rollers in the axial direction has to be devised, and such a scheme has been proposed (see Japanese Laid-Open Patent Publication (Kokai) No. H01-149081). Since the end temperature increases when a recording material passes through the nip section and decreases in the sheet-to-sheet interval, temperature increase at the ends of the fixing rollers can be prevented by keeping some interval between a precedent recording material and the next recording material (i.e. sheet-to-sheet interval) when recording materials are continuously passed through the fixing unit. According to the scheme proposed in the Japanese Laid-Open Patent Publication (Kokai) No. H01-149081, the throughput in continuous printing is fixed with respect to each recording sheet size, and when an increase in end temperature occurs during conveyance, the throughput is decreased so as to decrease the end temperature. Also, an increase in end temperature is suppressed by providing longer sheet-to-sheet intervals for narrower recording sheets.
FIG. 43A is a diagram showing states in which recording sheets are conveyed at different sheet-to-sheet intervals, and FIG. 43B is a diagram showing the number of recording sheets conveyed per minute with respect to each sheet-to-sheet interval. Here, the number of recording sheets conveyed per minute is expressed in the unit ppm (page per minute) and is referred to as the throughput. If recording sheets are equal in length, the throughput increases as the sheet-to-sheet interval decreases, and conversely, the throughput decreases as the sheet-to-sheet interval increases.
FIG. 44 is a graph showing a state in which the end temperature decreases as the sheet-to-sheet interval increases in continuous printing. In FIG. 44, sheet passages and sheet-to-sheet intervals are enlarged so as to make the explanation easier to understand. When a recording sheet is passed through the fixing unit, the end temperature increases, and in the sheet-to-sheet interval, the end temperature decreases. The end temperature is repeatedly changed in this manner to gradually increase. Also, the end temperature represented by the graph G with a shorter sheet-to-sheet interval reaches an upper limit (210° C.) earlier than the end temperature represented by the graph H with a longer sheet-to-sheet interval. Thus, the degree of increase in end temperature varies with sheet-to-sheet intervals.
FIG. 45 is a graph showing the relationship between the throughput and the end temperature. When the throughput is 20 ppm, the end temperature reaches the upper limit of 210° C. at a time TD1; 18 ppm, TD2; 16 ppm, TD3; and 14 ppm, TD4. Thus, as the throughput decreases, the ascending curve of the end temperature becomes slighter, and also, the time it takes for the end temperature to reach the upper limit of 210° C. increases.
In Japanese Laid-Open Patent Publication (Kokai) No. H01-149081 described above, continuous sheet conveyance is started at a fixed throughput suitable for the recording sheet size, and when the end temperature reaches an upper limit during the sheet conveyance, the throughput is decreased so as to prevent an increase in end temperature. According to this method, if the continuous printing number is small, continuous printing at a high throughput can be realized, but if the continuous printing number is large, the throughput has to be decreased during sheet conveyance. As a result, the average throughput in continuous printing as a whole is low.
FIG. 46 is a graph showing the relationship between the continuous printing number and the printing time. The ordinate indicates the continuous printing number, and the abscissa indicates the printing time. The slope of each line corresponds to the number of prints produced per unit time, i.e. the throughput. For example, since the end temperature reaches the upper limit (210° C.) at the time TD1 if printing is started at a throughput of 20 ppm, the throughput is decreased to 5 ppm after the time TD1. At a point a in FIG. 46, the throughput is decreased from 20 ppm to 5 ppm, and the slope of a corresponding line becomes gentle. If it has been found by experiment that when the throughput is decreased to 5 ppm, the end temperature decreases from 210° C. and becomes stable at a temperature lower than 210° C., the remaining recording sheets are conveyed at 5 ppm.
In the case where, for example, 99 prints are produced in the above described manner, printing on the 99th recording sheet ends at a time T1. The throughput from an origin point to the point a is 20 ppm, and the throughput from the point a to an ending point I is 5 ppm. If printing is started at a throughput of 18 ppm and printing on the same 99 recording sheets is carried out, the slope of a line representing the number of prints produced per minute is gentle. In this case, since the sheet-to-sheet interval is longer and the increase in end temperature is smaller than in the case where the throughput is 20 ppm, the end temperature reaches the upper limit of 210° C. at a time TD2 which is later than in the case where the throughput is 20 ppm. If the throughput is decreased to 5 ppm at a point b corresponding to the time TD2, the slope of a line representing the number of prints produced per minute is the same as in the case where the throughput is 20 ppm. Thus, printing on the 99th recording sheet ends at a time T2, which is earlier than the time T1. From then on, the throughput is decreased in the same manner, points at which printing on the 99th recording sheet ends are I, II, III, IV, and V in FIG. 46. Thus, if printing is started at 12 ppm, the line does not bend (i.e. the throughput does not decrease) until printing on the 99th recording sheet ends at the point V, and continuous printing on 99 recording sheets is completed at the earliest time T5. Thus, in continuously producing 99 prints, the time it takes to complete printing on all the 99 sheets in the case where the throughput is 12 ppm is shorter than in the case where the throughput is 20 ppm.
As described above, according to the above conventional art, recording sheets are conveyed in continuous printing at a fixed throughput which is determined with respect to each recording sheet size, and therefore, if the continuous printing number is large, the end temperature increases during sheet conveyance, and hence the throughput has to be decreased during conveyance so as to decrease the end temperature. As a result, it takes long time to complete printing, and it is impossible to control conveyance in the optimum manner with respect to each number of prints to be continuously produced, that is, it is impossible to control conveyance such that printing is completed within the minimum period of time. Specifically, according to the conventional art, a fixed and fastest throughput cannot be set until continuous printing is completed, since an increase in end temperature to exceed an upper limit has to be prevented.
Further, since the proper fixing temperature and the increase in end temperature increase varies with sheet types such as a thick sheet, a thin sheet, and an OHP sheet, the throughput has been controlled to be changed according to sheet type at the start of printing, a decrease in throughput during sheet conveyance cannot be avoided when the continuous printing number is large.